Mobile data traffic is growing exponentially due to the enormous success of smart phones, tablets and other data traffic appliances. One of the key difficulties in delivering high data rates is the natural variability of the radio propagation channels. Power control and adaptive modulation and coding are classical methods that adapt the signal quality and data rates to the current radio channel conditions. The may be used to combat channel fading to achieve a constant data rate. Alternatively, the instantaneous data rate may be adapted to the instantaneous channel fading, to send more data when the channel has favourable conditions and less when it is unfavourable. These methods may be applied both over time and over frequency subcarriers. The power and modulation adaptation need to be re-determined over the same time/frequency scale as the channel varies. Since substantial channel variations may occur over a few milliseconds and a few hundred kHz, this calls for cumbersome channel estimation and feedback mechanisms.
Network nodes equipped with a large number of antennas may simultaneously schedule multiple wireless devices, or terminals, at the same time/frequency band and communicate using simple linear processing such as Maximum-Ratio, MR, and Zero-Forcing, ZF. This is an attractive approach to handle the increasing data traffic, since it does not require a denser network deployment and since each network node may control the interference that it causes to its local area. Using many antennas at the network node along with appropriately chosen precoding results in an effective channel between the network node and the wireless device that is substantially independent of the small-scale fading and looks flat over frequency. This property is often referred to as channel hardening. Systems with many antennas are often referred to as massive multi-user Multiple-Input-Multiple-Output, MIMO, abbreviated by massive MIMO hereafter.
While the channel hardening property leads to effective precoded channels with very small time and frequency variations in terms of Signal to Noise Ratio, SNR, the received signals are also affected by interference from other cells or other systems, operating in the same or in adjacent bands. The interference power, or level, may vary rapidly over time and frequency, due to lack of channel hardening and/or fast resource allocation variations in the vicinity of the cell. This effect is sometimes known as “flashlight” interference. These variations cannot be measured at the network node; thus existing technologies require feedback mechanisms where the wireless device reports the interference level at different subcarriers. The feedback load is cumbersome since the interference may be highly frequency selective and time varying.